Xilinx Frequency Generator for Spartan-3E Starter Kit - Manual (Page 12)

Frequency Generator for the Spartan-3E Starter Kit 12
Final Output Spectrum
Observing the frequency spectrum of the final output reveals that you can not get something for nothing and helps us to understand when the frequency
aligned mode should and should not be used. I have used the infinite persistence display again and this time it was even more useful to do so. As before,
plots on the left cover up to up to 50MHz and on plots on the right show ±5MHz centred on 12.5MHz.
These plots show that the 12.5MHz signal is
actually not as good as that generated
directly at the output of the phase
accumulator. Although clearly centred at
12.5MHz the spectrum shows that there is
an increased bandwidth. This reflects that
the DCM is tracking the input frequency
even though it doesn’t really need to do
anything. It is rather like balancing on a wall;
we know the wall isn’t moving but we still
wobble a bit to stay balanced because we
are unable to freeze completely due to other
influences on us and the need to breath etc!
12.5MHz
Fundamental
5MHz/division
10dB/division
1MHz/division
10dB/division
3 rd
Harmonic
12.5MHz
Fundamental
12.4125MHz
Fundamental
12.4125MHz
Fundamental
The full spectrum of the 12.4125MHz case
shows how the previous ‘family of spectral
components’ associated with the 5ns of cycle
jitter have been removed and that the noise
floor has been returned to normal levels. The
zoomed plot now shows a fundamental with
what looks like modulation sidebands rather
than fixed spectral components at ±0.74MHz.
This again reflects average frequency
tracking as well as the removal of the 5ns
cycle jitter. Note that an agile frequency
component has a lower energy (W/Hz) than a
static component.
±1.3MHz
Hint – If you can synthesize a perfect waveform with a phase accumulator or other direct clock division circuit then it does NOT make sense to use a DCM in
frequency aligned mode. In all non integer division cases the DCM will dramatically help jitter performance but some frequency ‘tracking’ must be accepted.